A steel cord with different bending stiffness in different directions, i.e. lateral bending stiffness in horizontal plane and radial bending stiffness in vertical axis, has a good advantage in comparison with a normal steel cord with same bending stiffness in the different directions. While applying such steel cord in rubber product, i.e. rubber tyre, the different bending stiffness in different direction leads to a good handling, riding comfort or driving stability. In a direction perpendicular to the horizontal axis of the steel cord, there is a decreased radial bending stiffness and an increased flexibility so that any obstacles such as cobble stones on the road can be taken easily. In the direction of the horizontal axis of the steel cord, there is an increased lateral bending stiffness and a decreased flexibility leading to an improved stability when taking a bend.
Normally the present steel cord with different bending stiffness in different direction is produced as the steel cord with oval cross-section or rectangular cross-section.
U.S. Pat. No. 4,544,603 discloses a steel cord having a lateral bending stiffness in a horizontal plane different from the radial bending stiffness in a vertical plane. This steel cord has several steel filaments overlapped with each other. The steel filaments are flat wires having substantially rectangular cross-section with round edges. Because the steel filaments have the same cross-section dimension, the steel cord has a cross-section of rectangular form. While embedding the steel cord into rubber for reinforcing rubber product, this rectangular cross-section has a disadvantage. When the steel cord is embedded into the rubber, use is made of an extruding or calendaring operation. This extruding of calendaring operation makes use of an extrusion die which is mostly round. In other words, the cross-section of hole in the extrusion die is round. Now, the steel cord with a rectangular cross-section can not fit the extrusion die with round cross-section. As a result, the steel cord will turn over or twist during the extrusion or calendaring process, and the rigid edges of the steel cord will damage the extrusion die. An optional solution is using other extrusion die with a rectangular cross-section. Because this rectangular extrusion die is not a normal or general die, it is more expensive than round die, the solution of using extrusion die with a rectangular cross-section will increase the production cost.
U.S. Pat. No. 4,606,392 also discloses a steel cord for reinforcing element comprising several flat wires twisted together. Because of this twisting of the steel cord, the steel cord has a substantially round cross-section. The steel cord can fit the normal extrusion die with round cross-section, but the difference between the lateral bending stiffness and radial bending stiffness is not existing. While using the rubber tyre with such steel cord, the handling and riding comfort is not different from other steel cords with round cross-section.